Do You Have to Kill the Power?

A Chief Officer is asking some challenging questions about powering down a vehicle's 12-volt electrical system at a crash scene.

Below are his questions and my reply. How would you answer this if you were a fire officer and someone in your department asked these questions?

"As a chief I am looking for a more professional opinion on this matter. On "typical accidents" that involve entrapment or confinement how important is it to de power a vehicle. Does it "have to be done"?

My REPLY;
Powering down the 12-volt power on a vehicle involved in a crash is important but the situation at hand does have to be taken into consideration.

If the crash-damaged vehicle is unoccupied, then a fire department officer on-scene can decide if anything needs to be done to the 12-volt electrical system of the vehicle at all. If in doubt, the officer should have a crew open the hood normally and disconnect both the ground and the hot cables from the vehicle's battery.

If EMS personnel are going to be working with a patient inside the vehicle, the fire/rescue team officer should then assign a crew to the electrical system shutdown. Disconnect the 12-volt power cables if they is readily accessible. The crew can 'double cut' the cables to shut down the power if access to the battery is obstructed. This is justified due to the risk posed to responders from items such as undeployed airbags for example.

I recommend that anytime a fire/rescue crew commits vehicle rescue tools to a vehicle for extrication tasks, the electric system of that vehicle should be shut down prior to their work. Ripping off a door, cutting a roof, moving the dash, etc, are all tasks that you do not want arcs, sparks, or electrical shorts to occur during the work.

I got one addition: The first step of the procedure should always be to switch the vehicles ignition to off. This has several reasons, among others, switching off the ignition is important to prevent accidental start of the vehicle and airbag deployment (the SRS sensing system is normally only able to work properly with the ignition in the on position). Furthermore it will open the contacters in xEVs which is important because shuting down 12 Volt alone may not disable the high and low voltage system as long as the ignition is in the on position.

Expanding on what Jorg said, not only should you turn the vehicle off but take the keys physically out of the car. Many newer cars have RFID chips and can activate the 12-volt system from distances of at least 6'. Taking the keys also prevents someone from putting them back into the ignition without everyone being aware.